Bridled Tern Onychoprion anaethetus 褐翅燕鷗
Category I. Summer visitor that breeds on offshore islets in eastern waters and an uncommon passage migrant mainly in autumn; large numbers can occur during tropical storms.
IDENTIFICATION
May 2019, John and Jemi Holmes.
30-32 cm. As the only regularly-occurring large tern in HK that is dark above, it is generally straightforward to identify birds here. It has a long tail with a deep fork, whitish outer tail feathers and relatively long and narrow wings.
Apr. 2018, Michelle and Peter Wong.
On the adult upperparts are dark brownish-grey, slightly paler on the upper mantle and rump, and the crown, nape, hind neck, lores and bill are black, isolating the white forehead that extends behind the eye; the underparts are white. The black lores (‘bridle’) clearly join the bill on the upper mandible.
Aug. 2021, Michelle and Peter Wong.
Juveniles are paler above, have a poorly marked crown and lores, and pale fringes to the mantle, scapulars and wing coverts.
VOCALISATIONS
Calls are generally deeper and more nasal than those of other terns in HK.
DISTRIBUTION & HABITAT PREFERENCE
Occurs in all sea areas apart from those subject to significant influence from estuarine waters (there are no records from Deep Bay, for example). Records in the breeding season are concentrated around the breeding sites, while in earlier in spring and in autumn they are seen in more widespread areas, in particular southern waters.
OCCURRENCE
The first record concerned six birds in the tern colony on Shek Ngau Chau (then also known as Gau Tau), Mirs Bay on 7 July 1985 (Chalmers 1987). Breeding was strongly suspected in 1985 and subsequent years but was not proven until July 1993 when a nest was found in a colony of Black-naped and Roseate Terns on Tung Kwo Chau. Although successful breeding occurred during the period 1996-1998 and despite large numbers of adults sometimes being present, at the time Bridled Tern was considered to retain a precarious toehold as a breeding bird in HK due to its erratic presence.
The earliest spring record is of four off Po Toi on 6 April 2011, and the highest count in that month is 71 on the 28th. Although all April records appear to be of migrants, a count of 150 on Shek Ngau Chau on 6 May 2000 indicates that birds can arrive at the breeding site early in May. For details of occurrence at breeding sites see below.
It appears that from at least early September few birds generally remain at the breeding sites and the distribution of records reverts to that in April, with most birds seen in eastern and southern waters. The close approach of tropical storms can influence the number of birds seen, and the highest such autumn count is of 90 on 26 September 1999. The latest autumn record is four off Cheung Chau on 7 October 2019.
A chick ringed at Shek Ngau Chau on 11 August 1997 was re-trapped almost 21 years later at the same site on 27 July 2018.
BEHAVIOUR, FORAGING & DIET
Appears to carry out plunge-diving infrequently, preferring to swoop down from low heights to the water to take prey at or just below water surface. Foraging groups tend to spread out over a large area and not concentrate in the manner of other terns. Recorded eating small fish.
BREEDING
Since 2011 annual surveys have been carried out of the breeding population of breeding terns in HK. Surveys are made from moving boats that are often unable to make a close approach due to sea conditions, which makes it difficult to collect accurate data on the number of pairs or juveniles on or near the nest. From 2011 to 2021 the peak annual count of birds at breeding colonies varied from 468 to 804 (mean 630) (AFCD in litt.).
The nesting period lasts from mid-May to mid-August. Traditionally the highest counts were generally made at Shek Ngau Chau and The Ninepins.
RANGE & SYSTEMATICS
Breeds in tropical and subtropical seas in the Caribbean, west Africa, the Middle East, Indian Ocean and southeast Asia from Taiwan to Malaysia, Indonesia, northern Australia and Melanesia. Northerly and southerly populations are migratory (Haney et al. 2020). In China a locally common breeding summer visitor to coastal areas from Shanghai south (Liu and Chen 2020).
Four subspecies are recognised, of which the nominate occurs in the eastern part of the Indian Ocean and the western Pacific and is presumed to occur in HK.
CONSERVATION STATUS
IUCN: Least Concern. Population trend unknown.
Chalmers, M. L. (1987). Records Committee Report 1985. Hong Kong Bird Report 1984/1985: 32-46.
Haney, J. C., D. S. Lee, and R. D. Morris (2020). Bridled Tern (Onychoprion anaethetus), version 1.0. In Birds of the World (S. M. Billerman, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.briter1.01
Liu, Y. and Y. H. Chen (eds) (2020). The CNG Field Guide to the Birds of China (in Chinese). Hunan Science and Technology Publication House, Changsha.
